Numerous methods are known for producing metallic components for experimental or productive uses. For example, to produce an exhaust valve designed for use in an internal combustion engine, a typical manufacturing sequence may entail casting a machining blank by pouring molten metal alloy into a uniquely designed casting mold. Alternatively, a preformed metal slug may be alternately heated and forged using a series of progressive forming forging dies to produce a near net-shape forging. The forging or casting may then undergo numerous machining and heat treating steps necessitated by both the close geometric tolerances required and the harsh, high temperature, high stress operating conditions experienced in the engine.
While such manufacturing sequences may be economically justified for high volume production of components, small lot size or experimental quantities rarely justify the expense and time required to produce costly forging dies or unique casting molds. Often, where only a few components are required, or where the design of the components is changing iteratively in a design development cycle, the components are machined directly from blocks of metal in which relatively large quantities of excess material are removed through machining. Depending on the complexity of the component, this method is also often quite expensive, requiring both development of specialized tooling for fixturing the component during the machining cycles and highly skilled machinists for operating the requisite machine tools. Additionally, the fully machined component may resemble the desired design only insofar as certain features thereof which are required to establish geometric fit and function. Other important features, such as chemical composition and weight minimization may be deemed of secondary importance or too costly to achieve for experimental quantities, especially if the material is difficult to machine.
There exists a need to be able to produce metallic components rapidly and at reasonable cost in lot sizes as small as a single, unique component. Techniques of rapid prototyping, conventionally referred to as solid freeform fabrication (SFF), are known and systems are commercially available capable of producing prototypes which geometrically mimic a desired component, to varying degrees of geometric tolerance. Such techniques, including stereolithography, selective laser sintering, solid ground curing, three-dimensional printing, fused deposition modeling and recursive mask and deposit process, typically build a prototype by layers under computer control, using resins, ceramic or metal powders, photopolymers, coated paper, or thermoplastics.
While these techniques produce a geometric mimic of a desired component, potentially useful for checking for interference in assembly and for proper mating with abutting components, the prototypes are typically unsuitable for static or dynamic use under operational loads beyond the capability of the constituent material. For the case of the exhaust valve discussed above, it is contemplated that no conventional SFF technique could produce a component suitable for use in a running engine. For example, resin, paper and thermoplastic prototypes clearly lack thermal capability, and porosity severely compromises the structural integrity of prototypes formed from sintered metal or ceramic powders. Systems which purport to be capable of processing liquid metals are contemplated to have volumetric deposition rates so low, due to the inherent geometric instability of liquid metal and the resultant need for substantially instantaneous solidification, as to be altogether impractical for producing components of any substantial size within a reasonable time period, if at all.
Partially solidified metal slurries are included in a class of materials typically retorted to as semi-solid materials (SSMs) which have been advantageously employed in the production of net-shape components by injection molding. These SSMs are typically formed in billets and subsequently reheated to the semi-solid state for processing; however, rapid cooling and consequent premature solidification of the slurry in a mold may result in incomplete formation of a desired component. Relatively high shot velocities have been demonstrated to ameliorate the problem with some low melting temperature commercial alloys. A discussion of the technology, as applied generally to injection molding, is presented in "Net-Shape Forming Via Semi-Solid Processing," by S. B. Brown and M. C. Flemings, Advanced Materials & Processes, 1/93, pp. 36-40, the disclosure of which is herein incorporated by reference.